Programs from Steven Gill (gillsr@aston.ac.uk)
For the casio fx-9700GE (but prob work on all other models)
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.:    'print' character
>=    Greater than or equal to 
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Equation of tangent to f1 at X

Function in f1, input X value
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"TAN to f1 At X"
?->X
d/dx(f1,X)->A
f1-(AX)->B
"Y="
A.:
"X+"
B.: